The primary step any organism takes in order to sustain life involves controlling gene expression. The bacterium Escherichia coli produces a selenocysteine-containing enzyme, formate dehydrogenase, when grown under anaerobic conditions. We have used this as an easily manipulable model system for analyzing the regulation of gene expression at the transcriptional and translational levels. Fusions of the DNA sequences controlling formate dehydrogenase expression with the gene for an easily detectable enzyme marker, beta-galactosidase, have provided a simple assay system for factors affecting formate dehydrogenase synthesis. Recent reports have suggested that increased DNA supercoiling mediates the expression of certain anaerobic-specific genes (genes expressed only under anaerobic conditions) in facultatively- anaerobic bacteria. According to this model, expression of bacterial anaerobic-specific genes requires the activity of DNA gyrase, which adds negative supercoils to DNA. In contrast to the results found for other anaerobic-specific genes, it was found that inhibition of gyrase activity enhances the expression of formate dehydrogenase. Mammals, birds, and several species of bacteria incorporate selenium as selenocysteine at specific sites of a few proteins. The mechanism by which selenocysteine is incorporated into protein remains a mystery. In all known cases, the messenger RNA codon for selenocysteine is UGA, which normally is a stop codon terminating synthesis of the polypeptide chain. Recently, a tRNA was discovered in the laboratory of Dr. August Bock which recognizes a UGA codon and cotranslationally inserts a selenocysteine residue into the growing polypeptide chain. Using this tRNA, an in vitro system for translation of the formate dehydrogenase gene can be constructed, allowing elucidation of the biochemical machinery for selenocysteine incorporation.